The present invention relates to devices for sensing vibrations in earth formations. More specifically, the present disclosure is directed to electrodynamic sensing devices, such as geophones and seismometers, that have a moving coil placed in a magnetic field in a centered position. The present disclosure may be applicable to other types of vibration transducers, either in sensing or transmitting operation.
In the oil and gas industry seismic sensors are deployed at various locations, such as on the earth surface, in the sea, at the seabed, or in a borehole, to provide operationally significant subsurface structural and material information by measuring seismic signals reflected from changes in the subsurface structures. In this, seismic sensors are commonly used for purposes of obtaining useful data relating to acoustic impedance contrasts in subsurface structures.
Seismic sensors are also prevalent in earthquake monitoring, long term monitoring for water and CO2 reservoirs, nuclear test monitoring, and such like activity that require the accurate and efficient acquisition of seismic data.
In seismic signal detection, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors, and the output of the sensors used to determine the structure of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic signals from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. For example, the sensed seismic signals may be direct signals that are derived from micro-seismicity induced by fracturing or reservoir collapse or alteration, or reflected signals that are derived from an artificial source of energy.
Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense particle motion arising from a seismic source.
As depicted in FIG. 1A, a typical geophone 10 has one or more cylindrical moving coil 12 that is suspended by springs 20 so as to be disposed around a magnet 15 having pole pieces 16. The geophone 10 has a housing 14 and end caps 18. Each moving coil 12 is maintained at a neutral, rest position by the springs 20, and is free to oscillate in a magnetic field of the magnet 15 from a centered position thereof. Springs 20 are usually made with a sheet metal designed to maintain the coil 12 at a centered, equilibrium position relative to the magnetic field of the magnet 15. In a geophone that is designed for vertical operation, the springs 20 are pre-stressed to centralize the moving coil 12 in the magnetic field against gravitational acceleration.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface, in the sea or at the seabed, or on the wall of a borehole which penetrates the earth, moves with the particle motion caused by acoustic wave propagation.
If the axis of the geophone is aligned with the direction of motion, however, the moving coil mounted on the spring inside the geophone stays in the same position causing relative motion of the coil with respect to the housing. When the coil moves in the magnetic field, a voltage is induced in the coil which can be output as a signal.
FIG. 1B is a schematic depiction of a geophone in which x0 is the neutral position of the moving coil, x is the position of the coil in motion and ξ is the relative displacement of the coil against the center of the magnetic field. The spring and mass system creates a natural frequency, ω0=√{square root over (k/m )}, where k is the spring constant and m is the moving mass of the coil assembly. The movement of the moving coil relative to the magnetic field generates an electric output
      e    g    =            S      0        ⁢                  ⅆ        ξ                    ⅆ        t            where S0 is the sensitivity and
      ⅆ    ξ        ⅆ    t  is the velocity of the coil above the natural frequency of the geophone. The generated electric signal flows through the shunt resistor Rs and coil. The current i in the coil damps the movement of the coil. Exemplary amplitude and phase responses of a typical geophone with 10 Hz natural frequency are shown in FIG. 1C for different damping factors D.
In seismicity monitoring, it is desirable to acquire low frequency seismic data. In this, the signal-to-noise ratio (SNR) is improved if seismicity is measured in deep boreholes. However, the environmental temperatures in deep boreholes are often high, and the borehole diameters are small. It is difficult to design low frequency and high temperature geophones having a small size. In addition, geophones for borehole applications should be rugged because of rough handling. Borehole geophones are also expected to work under tilt since a borehole can be deviated. Furthermore, after installation in a deep hole a geophone may be required to continuously monitor seismicity for many years. The geophone is expected to function reliably for a long time at high temperatures.
As previously mentioned, the force due to gravitational acceleration is balanced with the natural displacement of the spring.mg=kξ0  Equation 1
where m is the moving mass; k is the spring constant, and ξ0 is the natural displacement. Sinceω0=√{square root over (k/m)}, ξ0=g/ω02  Equation 2Equation 2 shows that the natural displacement is inversely proportional to square of the natural frequency and the amount is large for a low natural frequency geophone.
FIG. 2A shows the natural displacement ξ0 of a geophone moving coil due to gravitational acceleration as a function of natural frequency f0=ω0/2π. This is the amount of pre-stress that is needed in the springs so as to centralize the coil. As evident from FIG. 2A, a pre-stress of about 2.5 mm is required for a 10 Hz geophone. In this, a large natural displacement is required for low frequency geophones, i.e., geophones having low f0. As discussed in more detail below, it is difficult to obtain low frequency seismic sensors having a small, compact size that is suitable for downhole deployment in a borehole.
Since low frequency signals are desired for some applications, such as inverting seismic signals to acoustic impedance or computing source mechanism from measured seismic signals, the natural frequency of the geophones needs to be reduced. However, the natural displacement is increased when the natural frequency of the geophones is lowered. A geophone is typically 1 inch in outer diameter and 1.2 inch in height. In such a small size, the maximum pre-stress that is possible in the spring is a few millimeters, but not in the order of inches. Even for a large seismometer, having, for example, an outer diameter of 2 inches and a height of 4 inches, it is still difficult to provide pre-stress of a few inches. Thus, it is difficult to design a geophone with a lower natural frequency and having a small size.
Typically, the springs of a geophone are made of beryllium copper for durability. The springs are designed to maintain the moving coil at the center of the magnetic field; however, over time there is creep in the spring. The creep is pronounced especially at high temperatures, as shown in FIG. 2B. It is known that inconel creeps less than beryllium copper, as shown in FIG. 2B. However, inconel is expensive and it is difficult to make an inconel spring.
Since spring creep causes the coil to be displaced from the center over time the geophone response also changes. Eventually, after a period of use, the moving coil may be displaced to the bottom of the geophone housing, and the geophone will not respond to external vibration.
If a geophone is tilted, i.e., is moved away from the orientation that it is designed for, the moving coil is eccentered with respect to the magnetic field in the magnet. Note FIG. 2C. As mentioned above, the springs that support the moving coil are pre-stressed to compensate for gravitational force so that the moving coil is centered in the geophone. However, if such a geophone is tilted, the pre-stressed springs cause the moving coil to move in the upward direction. Therefore, the moving coil is displaced from its neutral position relative to the vertical position of the geophone, as depicted in FIG. 2C. In FIG. 2C, the neutral or rest position of the moving coil of a vertical geophone seismic sensor is designated as x0, and the displaced position due to tilt θ is designated as x. If the amount of tilt is large, the moving coil may hit an end cap of the geophone so that the geophone is no longer able to respond to seismic vibrations.
When a geophone is tilted from the vertical orientation, the pre-stressed springs move the moving coil in an upward direction by ξ due to less gravitational acceleration, as shown in FIG. 2C. The amount of ξ is:
                              ξ          V                =                              g                          ω              0              2                                ⁢                      (                          1              -                              cos                ⁢                                                                  ⁢                θ                                      )                                              Equation        ⁢                                  ⁢        3            where g is gravitational acceleration; θ is tilt measured from vertical; ω0 is angular natural frequency equal to 2πf0; k is spring constant; and m is moving mass. The subscript v denotes a vertical geophone having a spring that is pre-stressed to center the moving coil when the geophone is vertical.
Similarly, for a horizontal geophone without a pre-stressed spring, the dislocation of the moving coil is:
                              ξ          H                =                              g                          ω              0              2                                ⁢          sin          ⁢                                          ⁢          θ                                    Equation        ⁢                                  ⁢        4            The natural displacement is large when the natural frequency of the geophone is low, as shown in FIG. 2D, and the shift of natural displacement due to tilt is large. Since the stroke of the moving coil is finite by design, the moving coil may exceed the maximum space for it to move and the geophone will stop responding to vibrations.
When the moving coil is not centered in the magnetic flux field, the open circuit sensitivity, S0 and open circuit damping, D0 are reduced and total harmonic distortion becomes large. In this, if a vertical geophone is tilted from its vertical position the geophone response parameters So, Do, and fo change based on the amount of tilt. Changes in geophone response parameters change the waveform of recorded seismic signals, which is not desirable for the analysis of the recorded data.
In summary, in order to measure small seismic signals at low frequency, the natural frequency of a geophone should be low. However, when the natural frequency is lowered the natural displacement of the geophone becomes large, and the size of the geophone increases in order to accommodate the natural displacement of the geophone. When the natural displacement of the moving coil due to gravitational acceleration is large, it is difficult to accommodate the changes in natural displacement for different tilt conditions. Therefore, the range of tilt is very limited and precise setting of the geophone is required during installation. Finally, there is spring creep over time, especially at high temperatures, and such a geophone stops working after being used for a long period of time.
Accordingly, it will be appreciated that there exists a desire to improve upon conventional geophones in order to improve the accuracy of seismic measurements.
The limitations of conventional seismic sensor designs noted in the preceding are not intended to be exhaustive but rather are among many which may reduce the effectiveness of previously known sensor mechanisms. The above should be sufficient, however, to demonstrate that sensor structures existing in the past will admit to worthwhile improvement.